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Register a new userLongitudinal Spin Transfer in Inclusive $Lambda$ and $bar Lambda$ Production in Polarized Proton-proton Collisions at $sqrt s$ =200 GeV
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Qinghua Xu
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This contribution reports on a proof-of-principle measurement of the longitudinal spin transfer $D_{LL}$ in inclusive $Lambda$ and $bar Lambda$ production in polarized proton-proton collisions at a center of mass energy $sqrt{s}$ = 200 GeV. The data sample consists of about 3 $times$ $10^6$ minimum bias events collected in the year 2005 by the STAR experiment at RHIC with proton beam polarizations of up to 50%. The $Lambda$($bar Lambda$) candidates are reconstructed at mid-rapidity ($|eta|<1$) using the STAR Time Projection Chamber via the dominant decay channel $Lambdato p pi^-$ ($bar Lambda to bar p pi^+$). Their mean transverse momentum $p_T$ is about 1.3 GeV/c and longitudinal momentum fraction $x_F = 7.5 times 10^{-3}$ . The longitudinal $Lambda$($bar Lambda$) polarization is determined using a method in which the detector acceptance mostly cancels.
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The transverse spin transfer from polarized protons to $Lambda$ and $bar{Lambda}$ hyperons is expected to provide sensitivity to the transversity distribution of the nucleon and to the transversely polarized fragmentation functions. We report the first measurement of the transverse spin transfer to $Lambda$ and $bar{Lambda}$ along the polarization direction of the fragmenting quark, $D_mathrm{TT}$, in transversely polarized proton-proton collisions at $sqrt{s}=200,mathrm{GeV}$ with the STAR detector at RHIC. The data correspond to an integrated luminosity of $18,mathrm{pb}^{-1}$ and cover the pseudorapidity range $left|etaright| < 1.2$ and transverse momenta $p_{mathrm{T}}$ up to $8,mathrm{GeV}/c$. The dependence on $p_mathrm{T}$ and $eta$ are presented. The $D_mathrm{TT}$ results are found to be comparable with a model prediction, and are also consistent with zero within uncertainties.
The longitudinal spin transfer $D_{LL}$ to $Lambda$ and $bar{Lambda}$ hyperons produced in high-energy polarized proton--proton collisions is expected to be sensitive to the helicity distribution functions of strange quarks and anti-quarks of the proton, and to longitudinally polarized fragmentation functions. We report an improved measurement of $D_{LL}$ from data obtained at a center-of-mass energy of $sqrt{s}$ = 200 GeV with the STAR detector at RHIC. The data have an approximately twelve times larger figure-of-merit than prior results and cover $|eta|<$ 1.2 in pseudo-rapidity with transverse momenta $p_T$ up to 6 GeV/c. In the forward scattering hemisphere at largest $p_T$, the longitudinal spin transfer is found to be $D_{LL}$ = -0.036 $pm$ 0.048 (stat) $pm$ 0.013(sys) for $Lambda$ hyperons and $D_{LL}$ = 0.032 $pm$ 0.043,(stat) $pm$ 0.013,(sys) for $bar{Lambda}$ anti-hyperons. The dependences on $eta$ and $p_T$ are presented and compared with model evaluations.
Preliminary results for the longitudinal polarization of $Lambda$ and ${bar Lambda}$ hyperons in longitudinally polarized proton-proton collisions at $sqrt s$ = 200 GeV are presented. The $Lambda$($bar Lambda$) candidates are reconstructed at mid-rapidity ($|eta|<1$) with the time projection chamber of the STAR experiment at RHIC, using 0.5 pb$^{-1}$ collected in 2003 and 2004 with beam polarizations of up to 45%. Their mean longitudinal momentum fraction $x_F$ is about 8 $times$$10^{-3}$ and their mean transverse momentum $p_T$ is about 1.5 GeV. The analysis uses asymmetries of counts for different spin states of the colliding proton beams and does not require detailed knowledge of the detector acceptance. The preliminary $Lambda$(${bar Lambda}$) polarization values are consistent with zero within their statistical uncertainties of 0.05.
We report our measurement on longitudinal spin transfer, D_LL, from high energy polarized protons to $Lambda$ and $bar{Lambda}$ hyperons in proton-proton collisions at $sqrt{s} = 200 GeV$ with the STAR detector at RHIC. The current measurements cover $Lambda$, $barLambda$ pseudorapidity $|eta| < 1.2$ and transverse momenta $p_T$ up to $4GeV/c$ using the data taken in 2005. The longitudinal spin transfer is found to be D_LL= -0.03pm 0.13 (stat) pm 0.04(syst)$ for inclusive $Lambda$ and $D_{LL} = -0.12 pm 0.08(stat) pm 0.03(syst)$ for inclusive $bar{Lambda}$ hyperons with $<eta> = 0.5$ and $<p_T> = 3.7 GeV/c$. The prospects with 2009 data and the future measurements are also given.
We report high-precision measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for midrapidity inclusive jet and dijet production in polarized $pp$ collisions at a center-of-mass energy of $sqrt{s}=200,mathrm{GeV}$. The new inclusive jet data are sensitive to the gluon helicity distribution, $Delta g(x,Q^2)$, for gluon momentum fractions in the range from $x simeq 0.05$ to $x simeq 0.5$, while the new dijet data provide further constraints on the $x$ dependence of $Delta g(x,Q^2)$. The results are in good agreement with previous measurements at $sqrt{s}=200,mathrm{GeV}$ and with recent theoretical evaluations of prior world data. Our new results have better precision and thus strengthen the evidence that $Delta g(x,Q^2)$ is positive for $x > 0.05$.
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